The present invention relates to an electroinductive senser and more particularly to a means for detecting discontinuities in metallic test objects of arbitrary shape.
By discontinuities is meant here all types of cracks, recesses, cavities, enclosures of foreign particles or structural variations which may affect the quality of the metallic material to be tested.
There has for a long time been a need for the manufacture of electroinductive sensers for testing metal objects of arbitrary shape, for example billets of arbitrary profile. One problem with known electroinductive sensers has, however, made the construction of such electroinductive sensers impossible and this problem is the so-called "liftoff" problem. Briefly, the liftoff problem is that distance variations between test material and senser give such large variations in output signal from the senser that only very marked discontinuities in the material can be detected. The error-indicating signals are drowned by the liftoff signals. This problem is so great that even a slight ovality in a test material having circular section completely disrupts investigation of the material.
There are a number of solutions to this problem and as an example reference is only made to the device described in German published specification No. 1,423,980.